Anaerobic Fermentation Technology: Principles, Applications, and Future Trends

What is Anaerobic Fermentation?

Anaerobic fermentation, which is a common respiration process in all bacteria and eukaryotes, is a metabolic pathway that converts carbohydrates into organic acids, gases, or alcohols under anaerobic conditions. Anaerobic fermentation causes NADH to react with endogenous organic electron acceptors, extracting energy from the molecule and producing NAD+ and an organic product, such as ethanol, lactic acid, butyric acid, acetone, hydrogen, carbon dioxide, etc.

Fermentation normally occurs in an anaerobic environment. Microorganisms are engineered as cellular factories capable of efficiently manufacturing proteins, enzymes, flavor molecules, vitamins, pigments, and fats. In anaerobic fermentation, microorganisms overproduce valuable biological products such as secondary metabolites. It usually involves metabolic engineering to induce the production of certain metabolites by manipulating relevant metabolic genes of microorganisms.

Methods of Anaerobic Fermentation

There are three main fermentation methods: batch, fed-batch, and continuous fermentation.

Batch fermentation: By using closed vessels, all required mediums are added at the beginning of the fermentation process in order to maximize the production of microbial biomass as well as related metabolites.

Fed-batch fermentation: Fresh mediums are added during strain culture to prevent nutrient depletion as a limiting factor for microbial metabolite production. The fed-batch process is a partly open system that controls the productivity of microorganisms using different feeding strategies.

Continuous fermentation: This method is commonly used with industrial-scale production. The exponential period of microorganisms is extended by adding fresh nutrients to microbial cells and removing cells from the bioreactor at a specific rate and time. The three most common types of continuous culture are chemostat, turbidostat, and perfusion culture.

Anaerobic Fermentation Process

Raw Material Pretreatment

Organic materials are crushed, cut, or stirred to increase the contact area between the material and microorganisms, thereby improving fermentation efficiency. For materials with low moisture content, water should be added to adjust the moisture level to 55%-70%. For materials with high nitrogen content, a carbon source should be added to adjust the carbon-to-nitrogen (C/N) ratio to 20-30.

Filling and Sealing of Fermentation Equipment

The pretreated raw materials are evenly loaded into anaerobic fermentation devices, such as biogas digesters or fermenters, and should be filled as fully as possible to minimize residual air. After filling, the device must be strictly sealed to prevent air from entering and to create an oxygen-free environment for anaerobic microorganisms.

Fermentation Process

Hydrolysis and Acidification Stage: Under the action of anaerobic or facultative anaerobic microorganisms, complex organic substances such as cellulose, starch, and proteins are decomposed into small molecules like monosaccharides, amino acids, and fatty acids. The hydrolysis-acidification microbial clusters at the bottom of the fermenter are the primary participants in this stage.

Hydrogen and Acetic Acid Production Stage: Hydrogen- and acetic acid-producing bacteria further convert the small molecules produced during hydrolysis and acidification into acetic acid, hydrogen gas, and carbon dioxide. Acetic acid accounts for about 80% of the total organic acids, providing key substrates for the subsequent methanogenesis stage.

Methanogenesis Stage: Methanogenic archaea utilize acetic acid, hydrogen gas, and carbon dioxide to produce methane. Statistics show that about 70% of methane is derived from acetic acid decomposition, with the remainder synthesized from hydrogen and carbon dioxide.

Monitoring and Control of the Fermentation Process

Key Parameter Monitoring: It is necessary to monitor parameters such as fermentation temperature, pH value, biogas yield, and methane content in the biogas in real time. The optimal fermentation temperature for methanogens is around 35°C under mesophilic conditions and around 55°C under thermophilic conditions. The optimal pH range is between 6.8 and 7.5.

Control Measures: Based on the monitoring results, fermentation conditions should be promptly adjusted, such as maintaining the appropriate temperature using heating systems or adjusting the pH by adding acid or alkali.

Product Collection and Treatment

Collection and Utilization of Biogas: The generated biogas can be collected using devices such as water-sealed cabinets or gas bags. After purification processes like desulfurization and dehydration, the biogas can be used as fuel for power generation, heating, cooking, and other purposes.

Treatment and Utilization of Digestate and Slurry: The digestate and slurry are rich in nutrients and humic substances. They can be directly used as fertilizers or further processed into organic fertilizers, feed additives, or soil conditioners.

Process Characteristics

Microbial Diversity: A wide variety of microorganisms participate in aerobic fermentation, including bacteria, yeasts, molds, and actinomycetes. Different microorganisms play distinct roles at various stages, working synergistically to decompose and transform organic matter.

Abundant Metabolic Products: In addition to carbon dioxide and water, aerobic fermentation produces various organic acids, alcohols, esters, enzymes, and other metabolic products. These compounds have different uses and values, and can be utilized in the production of food additives, pharmaceutical intermediates, biofuels, and more.

Fast Reaction Rate: Due to the high energy efficiency of aerobic respiration, aerobic fermentation proceeds relatively quickly, with shorter fermentation cycles, typically completed within several hours to a few days. This makes it suitable for large-scale industrial production.

Continuous Oxygen Supply Required: Aerobic fermentation requires a high level of oxygen supply. Continuous aeration of oxygen or air into the fermentation system is necessary to maintain microbial aerobic respiration. Without sufficient oxygen, the fermentation process would be impaired or even fail.

Anaerobic fermentation Equation

Below are some common chemical equations for anaerobic fermentation:

Alcohol Fermentation

In the anaerobic environment of yeast cells, glucose is broken down into pyruvic acid, which is further reduced to ethanol and carbon dioxide. The reaction equation is:

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2ATP

For example, in winemaking, yeast ferments glucose in the absence of oxygen to produce ethanol and carbon dioxide.

Lactic Acid Fermentation

Lactic acid bacteria ferment glucose into lactic acid under anaerobic conditions. The reaction equation is: C₆H₁₂O₆ → 2C₃H₆O₃ + 2ATP

This process is commonly seen in the production of yogurt, where lactic acid bacteria convert lactose in milk into lactic acid.

Methane Fermentation

Methane fermentation is a complex process involving multiple stages and microbial communities. Organic matter is first hydrolyzed and acidified into small molecules such as fatty acids, alcohols, and hydrogen. These intermediate products are then converted into methane and carbon dioxide by methanogens. The main reactions are:

Hydrolysis and acidification stage:

Carbohydrates: C₆H₁₂O₆ → 2CH₃CH₂OH + 2CO₂

Proteins: Proteins → amino acids → NH₃ + organic acids

Lipids: Lipids → glycerol + fatty acids

Acetogenesis and hydrogen production stage:

Volatile fatty acids (e.g., propionic acid) are converted into acetic acid and hydrogen:

CH₃CH₂COOH + H₂O → CH₃COOH + 2H₂ + CO₂

Ethanol is converted into acetic acid and hydrogen:

CH₃CH₂OH + H₂O → CH₃COOH + 2H₂

Methanogenesis stage:

Acetic acid is converted into methane and carbon dioxide:

CH₃COOH → CH₄ + CO₂

Hydrogen and carbon dioxide are reduced into methane:

4H₂ + CO₂ → CH₄ + 2H₂O

Other Anaerobic Fermentation Reactions

The anaerobic decomposition of cellulose into methane and carbon dioxide:

(C₆H₁₀O₅)ₙ + 3nH₂O → 3nCH₄ + 3nCO₂

The anaerobic decomposition of proteins into ammonia, methane, carbon dioxide, etc.:

CₙHₐO₆Nₐ + (n + m)H₂O → (n + m)CH₄ + (n + m)CO₂ + dNH₃

The anaerobic decomposition of lipids into methane, carbon dioxide, etc.:

C₅₇H₁₀₄O₆ + 57H₂O → 57CH₄ + 57CO₂

The specific chemical equations for anaerobic fermentation vary depending on the type of organic matter being fermented and the microbial community involved. Below are some additional examples:

The anaerobic decomposition of glucose into methane and carbon dioxide:

C₆H₁₂O₆ + H₂O → 3CH₄ + 3CO₂

The anaerobic decomposition of acetic acid into methane and carbon dioxide:

CH₃COOH → CH₄ + CO₂

The anaerobic decomposition of hydrogen and carbon dioxide into methane and water:

4H₂ + CO₂ → CH₄ + 2H₂O

What is the difference between aerobic and anaerobic fermentation?

The primary difference between aerobic fermentation and anaerobic fermentation lies in their energy production processes, oxygen dependence, and byproducts. Here's a detailed comparison:

Advantages and Limitations of Anaerobic Fermentation

Common Anaerobic Fermentation Products

Ethanol: (e.g., from yeast in alcohol production)

Lactic Acid: (e.g., from bacteria in yogurt or muscle cells during intense exercise)

Acetic Acid: (e.g., from acetogenic bacteria)

Butyric Acid: (e.g., from Clostridium species)

Propionic Acid: (e.g., from Propionibacterium in Swiss cheese production)

Gases:

• Carbon dioxide (CO₂)
• Hydrogen gas (H₂)
• Methane (CH₄) (from methanogenic archaea)

Application Fields

Food Industry: Used for the production of fermented foods such as yogurt, cheese, soy sauce, and vinegar, imparting unique flavors and nutritional value to foods. It is also applied in making bread, beer, and wine, improving the quality and taste of products.

Pharmaceutical Industry: Applied in the production of pharmaceutical products such as antibiotics, vitamins, amino acids, and enzyme preparations. Examples include the fermentation of Penicillium to produce penicillin and the fermentation of yeast to produce B vitamins.

Biofuel Production: Biomass can be converted into bioethanol and other biofuels through aerobic fermentation, providing renewable energy sources.

Our Fermentation Capabilities & Facilities